Internal combustion engines comprise inlet and exhaust valves to control the flow of gases into and out of the combustion chamber of each engine cylinder. Ordinarily the valves are mechanically controlled by means of a camshaft. Profiled cams on the camshaft are used to control timing of opening and closing of each valve.
Since the physical shape and profile of the cams may only be optimised for one particular operating condition of the internal combustion engine, it is known to use a variable valve actuation system where the operation of the valves may by adjusted to suit changing demand. For example, a standard profile may be utilised during periods of medium or high engine demand and a late inlet valve closing (LIVC) profile may be utilised for the inlet valve during steady state conditions when the demand on the internal combustion engine is relatively low. In a LIVC profile the closing of the intake valve at about the end of the intake stroke is delayed, so that the intake valve remains open for a portion of the compression stroke. This results in a lower pressure within the cylinder. Consequently, the cylinder piston does less work during the compression stroke which leads to improved fuel efficiency.
A problem with use of an LIVC profile is that when instantaneously switching from the standard profile to the LIVC profile there is a sudden drop in the air-to-fuel ratio within the cylinder. This leads to a rich mixture which tends to produce unwanted soot particulates and smoke. In order to attempt to overcome this problem it is known to employ a control system that gradually switches from the standard profile to the LIVC profile over relatively large number of engine cycles—typically around 20 cycles. However such control systems are complicated and expensive.